Colloidal, e.g. 35 to 250 m.sup.2 /g, boehmite (also referred to as pseudoboehmite, alumina monohydrate and aluminum oxide hydroxide), especially in a form capable of forming stable aqueous dispersions, is a very useful material. Such boehmite has been produced commercially, e.g. as a by-product of the Ziegler process for making primary alcohols. This is an alkoxide-based approach. A number of applications have been developed for this type of product, including frictionizing agent for paper bags and boxes, antistatic agent for carpets and textiles, binder for catalysts and special ceramics, and thixotropic agent and viscosity control for various suspensions.
Sol-gel processing of alumina ceramics has received much interest in the last several years as a potential method of producing sintered alumina with properties superior to those of traditional alumina ceramics. The hope has been that uniform distribution of controlled additives along with very small, uniform porosity (which is characteristic of sol-gel material) would permit densification at ultra-low temperatures and result in alumina ceramics with greatly improved properties.
One major problem has been that the alumina sols are prepared from microcrystalline boehmite, and that in transforming from boehmite to gamma to alpha alumina, a porous structure with very poor sintering characteristics results. One approach to solving this problem is disclosed in U.S. Pat. No. 4,314,827. The patent discloses using a second component (5-6% MgO) which apparently modifies the calcination and sintering characteristics and allows a very fine grained, tough microstructure to be achieved.
In the prior art, other attempts have been made to develop high density alumina ceramic parts. For example, an article entitled "Enhanced Densification of Boehmite Sol-Gels by Alpha-Alumina Seeding" by Kumagai and Messing (November 1984) discloses seeding boehmite sol-gels with alpha alumina and subsequently sintering to obtain high densities at 1200.degree. C., whereas unseeded gel had to be sintered at 1600.degree. C. In this work, the boehmite powder was formed by hydrolysis of aluminum alkoxide.
European patent application No. 152,768 discloses that the hardness and microstructure of aluminous abrasives produced from alumina gels are enhanced by the introduction of very fine alpha alumina seed particles in the amount of 1% or less.
The prior art processes require rather expensive processing to obtain the sol-gel precursor for the sintered grain. Therefore, there is a great need for an inexpensive process which can result in a high quality sintered product. The subject invention provides such a process which will permit the use of sintered alumina based on sol-gel processing to have wide application.